1. Field of the Invention
The invention relates to a liquid crystal composition and a liquid crystal display device. More specifically, the invention relates to a liquid crystal composition suitable for use in a liquid crystal display device driven in an AM (active matrix) mode, and a liquid crystal display device containing the composition.
2. Related Art
A liquid crystal display device (which is a generic term for a liquid crystal display device, a liquid crystal display panel and a liquid crystal display module) utilizes optical anisotropy, dielectric anisotropy and so forth of a liquid crystal composition, and as an operating mode of the liquid crystal display device, such various modes include a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a bistable twisted nematic (BTN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode, and so forth. Among these operating modes, it has been known that an ECB mode, an IPS mode, a VA mode and so forth are capable of being improved in viewing angle, while the conventional modes, such as a TN mode, an STN mode and so forth, have a problem therein. A liquid crystal composition having a negative dielectric anisotropy can be used in a liquid crystal display device of these modes.
As a liquid crystal composition having a negative dielectric anisotropy capable of being used in a liquid crystal display device of these modes, such compositions have been variously studied that contain a liquid crystal compound having 2,3-difluorophenylene, in which hydrogen on a benzene ring is replaced by fluorine.
The liquid crystal compound is a generic term for a compound having a liquid crystal phase such as a nematic phase, a smectic phase and so forth, and also for a compound having no liquid crystal phase but being useful as a component of a composition. The content ratio of the component is calculated based on the total weight of the liquid crystal composition. The liquid crystal compound herein is a compound represented by formula (A). The compound may be an optically active compound.

In formula (A), Rx and Ry are each independently a hydrogen, alkyl, alkoxy, alkoxyalkyl, alkoxyalkoxy, acyloxy, acyloxyalkyl, alkoxycarbonyl, alkoxycarbonylalkyl, alkenyl, alkenyloxy, alkenyloxyalkyl, alkoxyalkenyl, alkynyl, alkynyloxy, cyano, —NCS, fluorine or chlorine. These groups have approximately 10 or less carbons. In the group having approximately 1 to approximately 5 carbons, arbitrary hydrogen may be replaced by fluorine or chlorine, and the total number of the replaced fluorine and chlorine is approximately 1 to approximately 11. In formula (A), ring A is 1,4-cyclohexylene, 1,4-phenylene, pyrane-2,5-diyl, 1,3-dioxane-2,5-diyl, pyridine-2,5-diyl, pyrimidine-2,5-diyl, decahydronaphthalene-2,6-diyl, 1,2,3,4-tetrahydronaphthalene-2,6-diyl or naphthalene-2,6-diyl. In ring A, arbitrary hydrogen may be replaced by fluorine or chlorine. In ring A, the total number of the replaced fluorine and chlorine is approximately 1 to approximately 4. In the 1,4-phenylene, arbitrary one or two hydrogens may be replaced by cyano, methyl, difluoromethyl or trifluoromethyl. In formula (A), Y represents a single bond, —(CH2)2—, —COO—, —OCO—, —CH2O—, —OCH2—, —CF2O—, —OCF2—, —CH═CH—, —CF═CF—, —(CH2)4—, —(CH2)3—O—, —O—(CH2)3—, —CH═CH—(CH2)2—, —(CH2)2—CH═CH—, —(CH2)2CF2O—, —OCF2(CH2)2—, —(CH2)2COO—, —(CH2)2OCO—, —COO(CH2)2—, —OCO(CH2)2—, —CH═CH—COO—, —CH═CH—OCO—, —COO—CH═CH— or —OCO—CH═CH—. In formula (A), n represents 1, 2, 3 or 4.
JP 2005-35986 A and International Publication No.2005/000995 disclose examples of a compound having a chroman ring and a composition containing the compound. JP 2005-105132 and JP 2005-281560 A disclose a liquid crystal composition containing a compound having a chroman ring, which is the first component of the invention. The compositions disclosed in these publications do not consider the combination of the first component and the second component of the inventions and have room for improvement.
A liquid crystal display device having such an operating mode as an IPS mode and a VA mode still has a problem as a display device as compared to CRT, and it is desirable to improve the characteristics thereof.
The liquid crystal display device driven in an IPS mode or a VA mode is constituted mainly by a liquid crystal composition having a negative dielectric anisotropy. In order to further improve the characteristics, the liquid crystal composition preferably has the following characteristics (1) to (5), i.e., (1) a wide temperature range of a nematic phase, (2) a low viscosity, (3) a suitable optical anisotropy, (4) a large absolute value of dielectric anisotropy, and (5) a large specific resistance.
The temperature range of a nematic phase relates to the temperature range where the liquid crystal display device is used, and a liquid crystal display device containing a liquid crystal composition having a wide temperature range of a nematic phase as in item (1) has a wide temperature range where the liquid crystal display device can be used.
A liquid crystal display device containing a liquid crystal composition having a small viscosity as in item (2) has a short response time. A liquid crystal display device having a short response time can be favorably used for displaying a moving image. Furthermore, upon injecting the liquid crystal composition into a liquid crystal cell of the liquid crystal display device, the injection time can be reduced to improve the workability.
A liquid crystal display device containing a liquid crystal composition having a suitable optical anisotropy as in item (3) has a large contrast. A liquid crystal composition having an optical anisotropy that is smaller than a conventional liquid crystal composition is favorable for a liquid crystal display device having a large cell gap or a liquid crystal display device of a reflection type.
A liquid crystal display device containing a liquid crystal composition having a large absolute value of dielectric anisotropy as in item (4) reduces the threshold voltage, decreases the driving voltage, and reduces the electric power consumption.
A liquid crystal display device containing a liquid crystal composition having a large specific resistance as in item (5) increases the voltage holding ratio and increases the contrast ratio. Therefore, a liquid crystal composition having a large specific resistance in the initial stage and has a large specific resistance even after using it for a long period of time is preferred.